Accelerated Graphics Port Interface Specification

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AGP3.0 Interface Specification

Rev. 1.0

Minimum 8 Cycles

2 Cycles

Min 16 16

bit NOP Commands

^ sync cycle

8X transfers resum

A.G.P. CLK

SB_STBF

SBA[7:0]#

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Master must not glitch

Strobes for 6 cycles

Com

Update

* Since SBA has inverted polarity in AGP3.0, the logical value of Sync = FEh while the electrical value is LLLL LLLH.

Figure 2-5: SB_STB Stopping and Starting Sequence

The exact timing between the AD group calibration cycle and the start of SBA calibration is not important

as long as it is completed before the start of the next AD group calibration cycle. Since the smallest

possible calibration period is 4ms (see section 2.7.4), the master must ensure that SBA calibration is

completed within this time.

2.1.4.1.1 AD CALIBRATION WITH SBA STROBES STOPPED

The above description of SBA calibration by the master and target assumed that the SBA strobes

(SB_STBF and SB_STBS) were running when the AD calibration cycle occurred. Now we consider the

case where the master, for any reason, stops the strobes prior to or during an AD calibration cycle. The

following steps ensure proper calibration of the SBA buffers on both sides before the master restarts the

SBA strobes. Figure 2-6 should be used as reference.

1. If the SBA strobes are stopped prior to an AD calibration cycle GNT, the master has the

opportunity to restart them if the SBA sequence meets the requirements given in Section 2.1.4.1.

The master can resume normal SBA activity, by asserting the Sync cycle on the SBA (as shown

in Figure 2-5) as late as the 5

clock period of the AD calibration cycle (T5 in Figure 2-6), which

is 4 clocks after GNT is sampled active by the master. This provides sufficient time for the

master to detect the AD calibration cycle and prevent the resumption of SBA activity. Beyond T5,

the master must no longer issue a Sync cycle to restart the strobes until 16 clocks have

transpired which in this example is in T22.

If the SBA strobes are stopped in or after the AD calibration GNT cycle, then the master must

delay the restarting of the SBA strobes until 16 cycles after the AD calibration cycle is complete.

2. Once the AD calibration cycle is complete (T5 in example shown in Figure 2-6), the target will

have up to 4 additional clock cycles to test for SBA activity (presence of strobes or a Sync cycle

on SBA). This period of testing completes by T9 per this example. If SBA activity is detected, the

target waits for the strobe stopping sequence described in Section 2.1.4.1 in order to update the

SBA buffers.

3. If no SBA activity is detected by the end of the testing period following AD calibration cycle, the

master and target are given 12 clock cycles to calibrate the buffers in the SBA group. In this

example, this period ends in T21. During this time, the target must ignore the SB_STBx signals

since the master may glitch them during calibration.

4. Following this SBA calibration period, the master may assert Sync on SBA to restart the strobes

in the immediately succeeding clock cycle – T22.